1. Field of the Invention
The present invention relates to an image processing unit, a noise reduction method, a program, and a recording medium for reducing noise of image data.
2. Description of the Related Art
In many cases, image data picked up by image pick-up equipment such as scanners and digital cameras contain noise due to the characteristics of image pick-up devices, such as a dark current and a residual current. Therefore, processing for reducing the noise is required. However, even if a low-pass filter or the like is used to simply reduce the noise, an important element such as an edge necessary for humans to perceive is blurred, which results in degradation of image quality. In view of this problem, technologies for processing noise for each region in accordance with the characteristics of each region of image data have been proposed (see, for example, Patent Document 1 through 4).
Patent Document 1 discloses an image processing unit that performs calculation of an edge and sharpening processing for correcting a space frequency based on the size of the edge in mutually different signal spaces. According to this technology, an image is transformed into a multiplex resolution space to perform filter processing, which in turn makes it possible to realize noise elimination and visually natural emphasis with respect to the image in a desired frequency band without emphasizing noise.
Furthermore, Patent Document 2 discloses a technology for extracting the size and the direction of the gradient of an edge from image data and selecting a preset filter in accordance with the size and the direction of the gradient.
Furthermore, Patent Document 3 discloses a technology for discriminating an edge region from a flat region and applying different filter processes to the regions.
Furthermore, Patent Document 4 discloses a technology for not performing noise reduction processing with respect to a pixel determined as an edge and changing filter characteristics with respect to other pixels in accordance with a distance to the closest edge and the intensity of the edge.
However, since the technologies disclosed in Patent Documents 1 through 3 do not use edge information in the vicinity of a pixel to be processed (a notice pixel), appropriate noise reduction suitable for a neighboring edge structure (arrangement of pixel values) cannot be performed.
Furthermore, since edge information is generally extracted from an image from which noise has not been eliminated, it is not possible to assure that all the edge information is correct. Accordingly, as described in Patent Document 2 through 4, if the edge cannot be correctly extracted when noise is eliminated at one pixel position where edge information is detected, degradation of image quality, such as excessive edge blurring or insufficient noise reduction, occurs.    Patent Document 1: JP-A-2004-282511    Patent Document 2: WO02/005544    Patent Document 3: JP-A-2005-142891    Patent Document 4: JP-A-2005-117449